Identifying the wide diversity of extraterrestrial purine and pyrimidine nucleobases in carbonaceous meteorites

The lack of pyrimidine diversity in meteorites remains a mystery since prebiotic chemical models and laboratory experiments have predicted that these compounds can also be produced from chemical precursors found in meteorites. Here we report the detection of nucleobases in three carbonaceous meteorites using state-of-the-art analytical techniques optimized for small-scale quantification of nucleobases down to the range of parts per trillion (ppt). In addition to previously detected purine nucleobases in meteorites such as guanine and adenine, we identify various pyrimidine nucleobases such as cytosine, uracil, and thymine, and their structural isomers such as isocytosine, imidazole-4-carboxylic acid, and 6-methyluracil, respectively. Given the similarity in the molecular distribution of pyrimidines in meteorites and those in photon-processed interstellar ice analogues, some of these derivatives could have been generated by photochemical reactions prevailing in the interstellar medium and later incorporated into asteroids during solar system formation. This study demonstrates that a diversity of meteoritic nucleobases could serve as building blocks of DNA and RNA on the early Earth.


Amino acids and amines with their analogues in the meteorite extracts
To compare with the series of N-heterocyclic compounds (-C-N-C-), we also searched linear N-containing compounds (-C-NH2), namely amino acids and amines, in the same meteorite extracts.
We detected various compound peaks corresponding to amino acids, amines, and their alkylated homologs as unhydrolyzed fractions ( Supplementary Fig. 8). This observation is consistent with the amino acid and amine distributions in previous reports (amino acids in Murchison 1 ; amino acids in Tagish Lake 2 ; amines in Murchison 3 ; amines in Tagish Lake 4 ). The HPLC/HRMS method applied in this study was not optimized to separate structural isomers of amino acids and amines, however, this analytical procedure enables us to simultaneously confirm the presence of solvent-extractable cyclic and linear N-containing compounds 5 .
For comparison with cyclic N-containing compounds mentioned above, Supplementary   Figure 9 shows variations in the relative abundances of alkylated amino acid and amine homologs in the meteorite extracts with relevance to the number of carbon atoms in the alkyl groups under the above rough assumptions. The variation of amino acid homologs in Tagish Lake extract is distinct from that in the other two meteorite extracts ( Supplementary Fig. 9a), as similarly observed in alkylated imidazole homologs (Fig. 3). In contrast to amino acids and imidazoles, the relative abundances of alkylated amine homologs exponentially decreased with increasing the number of 3 alkylcarbons (from C4 to C7) ( Supplementary Fig. 9b). The distinct variation pattern of alkylated amines from that of amino acids and imidazoles might imply their different formation processes during the formation of the early solar system.

Are heavy isotopes suggestive of extraterrestrial origin?
The enrichment of heavy isotopes such as D ( 2 H), 13 C, 15 N refs.6,7 and presence of racemic mixtures of chiral molecules such as amino acids and sugars 8, 9 have often been used as evidence of their exogenous origin. Based on the analytical validation of N-heterocyclic compounds using an optimized method (e.g., LC × GC / IRMS) 10 , the quantity of nanomoles of nucleobase molecules is promising enough for individual compound-specific stable isotope measurements (e.g., δ 15 N of purine and pyrimidine nucleobases) in the context of theoretical values. Based on the concentrations of the detected nucleobases, more than 5 g of meteorite would be necessary for d 15 N-isotopic measurements.
However, even if nucleobases are extracted from 5 g or more of meteorites, as long as chromatographic separation from other coexisting species at the same retention time is not sufficient, reliable δ 15 N values could not be measured, as inferred for the δ 13 C of uracil in the Murchison extract 11,12 . A recent laboratory study demonstrated that 13 C-enriched organic molecules can be produced with 13 C-depleted materials by a kinetic isotope effect during parent-body-like processes, which suggests that heavy C isotope enrichment in extraterrestrial nucleobases may not be indicative of their extraterrestrial 4 origin 13 . Moreover, because nucleobases are generally not chiral, their origin cannot be identified using this criterion. In contrast, structural diversity, particularly pyrimidine nucleobases, would reasonably be considered an important criterion for suggesting their extraterrestrial origin 14 .

Possible scenarios for the synthesis of nucleobases proposed so far
The oligomerization of hydrogen cyanide (HCN) was proposed in the 1960s 15,16 as a possible pathway for the formation of purine nucleobases under prebiotic conditions and has since been substantiated by several experimental studies [16][17][18] . Although it is unclear whether HCCCN is present in the meteorite parent bodies, it has been detected 5 in comet 67P 22 .
In addition to the HCN-related route, a number of laboratory studies proved that nucleobases can be produced from formamide (NH2CHO) under the conditions on meteorite parent bodies [23][24][25][26][27] .
When NH2CHO was heated with meteorite powder at around 100 ºC, various kinds of organic molecules including nucleobases were identified in the product 25 . Of note, without meteorite powder, purine was the only product, suggesting a potential role of meteorite powder as catalysts for the synthesis of nucleobases in this experiment 27 . Although the detailed synthesis pathways are not well understood, it is likely that diaminomaleonitrile (DAMN) 26 is a precursor of nucleobase, similar to the case of the HCN-based pathway. Since NH2CHO has been identified in the interstellar medium 28 and comets 29 , it is straightforward that NH2CHO plays a role as a reactant for the synthesis of nucleobases in meteorites.
Nucleobases can also be synthesized from inorganic materials such as ammonium hydrogen carbonate (NH4HCO3) and water through the meteorite impact to the early ocean 30 . After the shockrecovery experiments with the pressures of the samples reaching to 4-7 GPa for 1 μsec, both cytosine and uracil were identified in the product 30 .          Table 3. Pyrimidines and purines in organic residues. Relative abundances of pyrimidine and purine molecules in organic residues produced by photochemical reactions of interstellar ice analogs (Reanalysis of the sample produced in Oba et al. 5 ).